The invention relates to a device for preparing a gas stream before feed of same to a mass spectrometer, in particular for determining isotope ratios, wherein the gas stream contains carrier gas and one or more analytes.
Mass spectrometers for determining the isotope ratios of gases analyze only pure simple gases, such as CO2 and N2. Most of the substances that are to be analyzed are not present as pure gases. Therefore the substances must be converted into pure simple gases. This conversion process is preferably carried out in a carrier gas stream. As carrier gas, in particular helium is used. This is an on-line method.
Only a limited amount of carrier gas or helium can be fed to the ion source of the mass spectrometer. Typically, the appliances for preparing the gases that are arranged upstream of the mass spectrometer operate with a higher carrier gas stream or helium stream than can be fed to the mass spectrometer. This applies in particular to element analyzers.
For solving this problem, the gas stream containing the carrier gas or helium on the one hand and the pure simple gas—the analyte—on the other can be divided, such that only a relatively small carrier gas stream or helium stream passes into the mass spectrometer, while the remaining part of the gas stream is disposed of. It is a disadvantage in this case that a relatively large subquantity of the analyte is also disposed of and does not pass into the mass spectrometer.
U.S. Pat. No. 5,012,052 discloses a method for isotope ratio determination. A sample passes with a hydrogen carrier gas into a gas chromatograph. Subsequent to the latter, a selectively active membrane having a palladium coating is provided at which the hydrogen carrier gas is separated. At the same time the hydrogen carrier gas is replaced by helium as carrier gas.
The purpose of the present invention is providing a larger subquantity of the analyte than previously.